1. Field of the Invention
The present invention relates to a non-aqueous electrolyte battery.
2. Description of the Related Art
In recent years, the use of non-aqueous electrolyte batteries such as lithium ion batteries which are compact, light, and have high energy density, has been increasing rapidly as power sources for portable electric apparatuses and the like. In such non-aqueous electrolyte batteries, lithium cobalt oxide is generally used as an active material of the positive electrode and an organic solvent is used as the solvent for the electrolyte solution. As a result, there is a possibility of an abnormality occurring in non-aqueous electrolyte batteries, for example, where the temperature inside the battery suddenly increases due to an increase in the temperature of the environment in which the battery is utilized, or due to overcharging of the battery. Accordingly, a variety of techniques have been examined in the conventional non-aqueous electrolyte batteries in order to enhance the safety of the batteries.
For example, a non-aqueous electrolyte battery having a safety mechanism that utilizes a shut-down function of a separator that separates the positive electrode from the negative electrode has been proposed as means for increasing the safety of the non-aqueous electrolyte battery. According to this shut-down function, the separator made of polypropylene or polyethylene, thermally contracts due to the melting point of polypropylene or polyethylene in the case where heat is generated inside the battery as a result of the abnormality. Such a thermal contraction makes microscopic pores, which are created in the separator and enable ion migration, close, so that the current is blocked, preventing it from flowing between the positive and negative electrodes.
It is desirable to block the current as soon as possible in order to enhance the safety of the battery in the case where heat is generated as a result of the abnormality. Thus, it is desirable to set the shut-down temperature of the separator at a temperature as low as possible. Here, in the present specification, the shut-down temperature of the separator indicates the temperature when the current between the positive and negative electrodes is completely blocked after the closure of microscopic pores, which are created in the separator for ion migration. Thus, the separator is formed of a combination of materials, wherein the ratio of polyethylene, having a melting point lower than that of polypropylene, has been increased in order to set the shut-down temperature of the separator at a low level.
However, when heat is applied, polyethylene has a low dimensional stability in comparison with polypropylene and, therefore, the above described separator has a low dimensional stability when heat is applied, and as a result, a large thermal contraction occurs in the case when the temperature of the battery increases, so as to create a gap inside the battery short circuiting the positive and negative electrodes via this gap. Thus, the battery is disadvantaged due to its diminished safety.
In addition, there is a problem when polyethylene has a lower film-breaking temperature than polypropylene. As a result, there is a case where the positive and negative electrodes short circuit when the separator breaks immediately after the operation of the shut-down function of the separator. Thus, the battery is disadvantaged due to its diminished safety. Here, in the present specification, the film-breaking temperature indicates the temperature where the separator contracts and breaks due to heat, causing the positive and negative electrodes to short circuit.
Thus, in order to eliminate the above described disadvantages, a non-aqueous electrolyte battery, having an insulating plate that fuses with the separator inside the battery, has been proposed for the purpose of preventing the short circuiting due to the contact made by the positive and negative electrodes which have been exposed as a result of the contraction of the separator (for example, Japanese Unexamined Patent Publication No. 5(1993)-74443).
In the non-aqueous electrolyte battery described in the above prior art, however, both ends of the separator are fused with the insulating plate and, therefore, the separator cannot be prevented from breaking due to the contraction of the separator, and in effect, a disadvantage is created when the battery short circuits.